Photoconductographic material and process of preparation



United States Patent O 3,432,406 PHOTOCONDU'CTOGRAPHIC MATERIAL ANDPROCESS OF PREPARATION Donald R. Eastman, Rochester, N.Y., assignor toEastman Kodak Company, Rochester N.Y., a corporation of New Jersey NoDrawing. Continuation-impart of application Ser. No. 45,949, July 28,1960. This application Sept. 25, 1964, Ser. No. 399,375 The portion ofthe term of the patent subsequent to Oct. 13, 1981, has been disclaimedUS. Cl. 20418 6 Claims Int. Cl. B44c 1/04 ABSTRACT OF THE DISCLOSURENovel photoconductographic elements are prepared by coating anelectrically conductive support with a mixture containing zinc oxide ina polymeric binder and contacting the surface of the coating with aferrocyanide or a ferricyanide salt.

This application is a continuation-in-part of Ser. No. 45,949 filed July28, 1960, now US. Patent 3,152,969.

This invention relates to a novel photoconductographic material and to amethod for preparing that material, and, more particularly, thisinvention relates to a novel photoconductographic material having asuperficial layer of zinc ferrocyanide or zinc ferricyanide, a novelmethod of preparing the photoconductographic material, and the use ofthat material in the preparation of lithographic printing lates.

P It is well known that lithographic printing plates may be prepared byan electrophotographic process whereby an electrically conductive imagepattern is formed in a photoconductive layer, by exposure to a lightimage, and then treated electrolytically, for example, as disclosed inUS. Patent 3,010,883, to cause the image areas to become ink-receptiveand the background to become ink-repellent. While this type of processproduces a usable printing plate, the plate is frequently not stable tolight or storage, and furthermore the processing of the plate requiresthe use of expensive and complicated solutions to develop the images ofdeposited materials such as silver or copper.

A more recent discovery is exemplified in US. Patent 3,085,051 whereinthe photoconductive element, prior to exposure to light, is coated witha transparent substance, which upon electrolytic treatment releases anopaque image-forming material. In this system, the coated transparentsubstance is an indium compound, which upon electrolysis is reducedcathodically to indium metal, and is deposited on the exposed andconducting areas of the photoconductive layer to produce a visible andink-receptive image of indium metal. The resulting lithographic printingplate has two principal disadvantages; namely, (1) the ink-Waterdiiferentiation is not satisfactory, and (2) the cost of manufacturingthe photoconductographic material and using it in a duplication processis too high, principally because of the use of indium.

It is an object of this invention to provide a novelphotoconductographic material which can be used to prepare copies havinggood image contrast. It is another object of this invention to provide anovel photoconductographic material which employs simpler and lessexpensive materials for its. electrolytic development. It is anotherobject of this invention to provide a photoconductographic material thatcan be used to prepare a lithographic printing plate exhibitingexcellent ink-water differentiation. It is still another object of thisinvention to provide processes for preparing such photoconductographicmaterial. Other 3,432,406 Patented Mar. 11, 1969 objects will appear inthe accompanying detailed description of this invention.

The above objects are accomplished by providing a photoconductiveelement comprising three components, 1) a conductive support layer, (2)a photoconductive central layer of zinc oxide-resin, and (3) asuperficial layer of zinc ferrocyanide or zinc ferricyanide. This photoconductographic element is prepared by treating the surface of aconventional zinc oxide-resin photoconductive layer with an acidsolution of potassium ferrocyanide or potassium ferricyanide whichreacts with zinc to for-m zinc ferrocyanide or zinc ferricyanide,respectively.

The photoconductive layer of zinc oxide-resin is the conventional oneused in xerography and photoconductography, comprising photoconductivezinc oxide mixed with a resin binder. The preferred resin is a 70/30styrene/ butadiene copolymer which optionally may be modified by thepresence of other suitable materials such as silicones or petroleumresins. One particularly desirable blend is of the abovestyrene/butadiene copolymer, 10% of a silicone and 10% of a petroleumresin fraction. Other organic resins are operable for this function as abinder for the zinc oxide. The resin should be transparent in thethickness employed, Water-resistant, of sufiicient molecular weight tobe film-forming, and electrically nonconductive. Examples of suchresin-s include polystyrene, cellulose nitrate, polyvinyl butyral,polyvinylidene chloride, chlorinated rubber, and rubber hydrochloride.The styrene/butadiene copolymer is preferred because it is inexpensiveand has the most desirable combination of properties for this invention.

The zinc oxide used in this invention is that known to those skilled inthis art as French Process zinc oxide.

The zinc oxide-resin mixture is normally a blend of three parts byweight of zinc oxide per part of resin. This ratio is not critical andmay vary from about 2:1 to about 10:1 although a ratio of about 3:1 ispreferred since it provides the optimum in photoconductivecharacteristics. The zinc oxide-resin layer may be prepared as aself-supporting layer or it may be coated on or impregnated into anothersupporting material such as paper, metal, glass, or plastic, which is,or may be treated to become, electrically conductive.

The superficial layer of Zinc ferrocyanide or ferricyanide may be addedto the photoconductive layer of zinc oxide-resin by any of a variety ofmethods. One procedure is to spray, dip, or spread the photoconductivelayer with an acid solution (at a pH of less than 4) of a ferrocyanideor a ferricyanide, preferably potassium ferrocyanide. This treatmentconverts the zinc oxide at the surface of the coating to thecorresponding acid-insoluble, alkali-soluble zinc ferrocyanide or zincferricyanide which hydrates and precipitates as a white solid havinggood adhesion to the photoconductive layer. This product can be driedand stored for subsequent use. It is stable indefinitely at normal roomtemperatures. Another procedure is to apply zinc ferrocyanide or zincferricyanide in the form of a slurry to the zinc oxide-resinphotoconductive layer. Still another procedure is to treat thephotoconductive layer with separate applications of a solution of asoluble zinc salt, e.g., zinc sulfate, and a solution of a solubleferrocyanide or ferricyanide, e.g., potassium ferrocyanide, causing adouble decomposition reaction producing zinc ferrocyanide as aprecipitate.

In the operation of this invention a light image is focused on thephotoconductive layer of zinc oxide-resin causing the zinc oxide tobecome conductive in the exposed areas. A solution of an electrolyte,such as an aqueous solution of sodium chloride, is placed in contactwith the superficial layer of zinc ferrocyanide or zinc ferricyanide andan electric potential applied between the solution andthe conductivesupport of the photoconductive layer, the solution being made positiveand the support negative. The conductive, light-exposed areas of zincoxide become cathodic, releasing hydroxyl ions from the electrolyte, andthe resultant alkalinity solubilizes the zinc ferrocyanide, producingzinc ions which, in turn, are plated out on the cathodic areas as zincmetal.

The electrolyte employed in the development step of the process of thisinvention is any good conductor of electricity which permits the flow ofions between the anode and cathode of the electrolytic circuit. Thepreferred electrolytes are aqueous solutions of salts, particularly thesalts of alkali metals and alkaline earth metals. Among these salts themost desirable ones are the alkaline earth carboxylates such as calciumacetate and magnesium acetate in their hydrate forms.

The following examples illustrate various embodiments of this invention,including the preferred procedures and materials. Parts and percentagesare based on weight unless otherwise indicated.

EXAMPLE 1 This example illustrates the preparation of an imagerecordingmaterial. A photoconductive element comprising a conductive support ofaluminum foil coated with a photoconductive layer of zinc oxide-resinwhich is dyesensitized. The zinc oxide-resin is a mixture of three partsof zinc oxide per part of a resin sold under the name Pliolite S-7.(Pliolite 8-7 is a 70/30 copolymer of styrene/butadiene.) This elementis swabbed under safelight conditions with an aqueous solutioncontaining potassium ferrocyanide trihydrate, the pH of the solutionbeing adjusted to 2.5 with 12 N sulfuric acid. The surface of theelement is then rinsed with distilled Water, air dried at roomtemperature, and placed in contact with a silver density step tablet(0.3 log E per step). The element and the step tablet is then exposedfor 5 seconds to 400 foot-candles of tungsten illumination and thendeveloped electrolytically by stroking the surface in a single pass at arate of 2 inches per second with a sponge saturated with a 5% aqueoussolution of calcium acetate monohydrate, while the sponge is held at anelectric potential of 60 volts positive DC with respect to the aluminumfoil of the photoconductive ele ment. A dense deposit of zinc, possiblymixed with some iron, covers the lightexposed areas only of thephotoconductive layer, thereby forming a sharp visible image.

When this procedure is repeated in every detail except that thephotoconductive material is not treated with a superficial coating ofzinc ferrocyanide, there is no visible image formed on thephotoconductive material after development.

EXAMPLE 2 This example illustrates the preparation of a lithographicprinting plate.

A photoconductive element similar to that described in Example 1 isprepared having an aluminum foil as a conductive support for adye-sensitized zinc oxide-resin photoconductive layer. In this instancethe resin is a blend comprising 80% pliolite S-7 (described in Example1), 10% Silicone SR-82, and 10% Piccopale. This element is then treatedwith a potassium ferrocyanide solution similar to that of Example 1except that the pH is adjusted to 2 with 12 N hydrochloric acid. Theresulting product is then rinsed, dried, contacted with a step tablet,and illuminated as in Example 1 except that the intensity of theillumination is 200 foot-candles. The illuminated element is developedelectrolytically in a frame development bath for 3 seconds in a 1%sodium chloride solution using the same potential as in Example 1. Athin dark layer of a zinc image forms on the exposed areas.

Frame development involves the use of a rectangular stainless steelframe with a base of Teflon fluorocarbon resin as an insulator, Theframe is placed over the exposed photoconductive element, electrolytesolution is placed inside the frame covering the photoconductiveelement, and the potential is applied with the positive electrode beingthe stainless steel frame and the negative electrode being the aluminumbacking of the photoconductive element.

The developed sheet is tested for operability as the plate of alithographic printing process by wetting the sheet with a fountainsolution of 1 part Repelex per 7 parts water (as described in US. Patent2,393,875), followed by inking the sheet using a plastic hand roller anda greasy lithographic ink (IPI Speed King Jet Halftone Black Litho Ink).The sheet is receptive to ink in the exposed areas and repellent to inkin the unexposed areas. The inked image is transferred to a copy sheetwith excellent duplication.

EXAMPLE 3 The procedure of Example 2 is repeated, except that a 2%solution of calcium acetate monohydrate is used in place of the 1%solution of sodium chloride as the electrolyte with the result thatthere is less shorting. Otherwise the results are substantially the sameas in Example 2.

EXAMPLE 4 The procedure of Example 2 is repeated with the exception thata 5% solution of magnesium acetate tetrahydrate is used in place of the1% solution of sodium chloride as the electrolyte and comparable resultsare obtained The foregoing description and examples are intended to beillustrative of various embodiments of this invention, which should notbe construed as being limited in any fashion other than as defined inthe following claims.

I claim:

1. A photoconductograph element comprising three layers, the centrallayer of which is a photoconductive layer comprising a mixture of zincoxide and a substantially transparent, water-resistant film-formingorganic resin binder, the lower layer of which is an electricallyconductive support, and the upper layer of which is a thin,light-transmitting stratum of a compound selected from the groupconsisting of zinc ferrocyanide and zinc ferricyanide.

2. A photoconductographic element comprising three layers, the centrallayer of which is a photoconductive layer, comprising a mixture of 2-10parts by weight of zinc oxide per part of styrene/butadiene copolymer,the lower layer of which is an electrically conductive support for saidcentral layer, and the upper layer of which is a thin,light-transmitting stratum of a compound selected from the groupconsisting of zinc ferrocyanide and zinc ferricyanide.

3. A three-layered photoconductographic element consisting essentiallyof a central layer comprising a mixture of about 3 parts by weight ofzinc oxide per part of a resin binder comprising at least by weight of a70/30 copolymer of styrene/butadiene, a lower layer of an electricallyconductive support, and a thin upper layer of zinc ferrocyanide.

4. The element of claim 3 in which said lower layer is a metal.

5. The process of preparing a photoconductographic element comprisingcoating an electrically conductive support with a photoconductivemixture of 2-10 parts by weight of zinc oxide per part of a substantialytransparent, water-resistant, film-forming, organic resin binder,contacting the surface of the coated support with an acid solution of acompound selected from the group consisting of ferrocyanide salts andferricyanide salts, removing any excess of said acid solution, anddrying the final prodnot.

6. The process of preparing a photoconductographic element comprisingcoating an electrically conductive sup- 5 6 port with a compositioncomprising about 3 parts by References Cited Weight of zinc oxide perpart of a resin binder comprising UNITED STATES PATENTS at least 80% byWeight of a 70/30 copolymer of styrene/ 3 010 883 11/1961 Johnson et a1butadiene, contacting the surface of said composition with 3:152:96910/1964 Eastman. a solution of an alkali metal ferrocyanide at a pH ofless 5 3,165,458 11/1965 Harrimam than 4 for a time sufficient to permitzinc ferrocyanide to form on the surface of said composition, removingany JOHN MACK, ry Examiner. excess of said solution and drying theresulting product. T TUFARIELLO, Assistant Examine.-

